Do I have to add a List ID?
Yes, you do. The list ID is mandatory because it determinate wich list o receivers the user will be added.

Do I have to add a Group ID?
Add a Group ID is not mandatory. If you want to add user to a specific group of receivers, you can do it by adding a Group ID.

Can I subscribe user to my newsletter service without ask him for confirmation?
No, you can’t. It is considered spam! Please, use one of the options provided by MailUp Auto Subscription to let user decide to receive or not your news.